1. Field of the Invention
The present invention elates to an image processing method and apparatus utilizing reduction/enlargement and compression/expansion of an image, which method and apparatus can be applied to an output apparatus such as a printer.
2. Related Background Art
A conventional image processing apparatus for outputting and printing an image must be prepared with a bit map memory having a resolution corresponding to an output resolution of the image. On the other hand, along with development of recent printing apparatuses, a high-resolution output is required, and the capacity of a bit map memory inevitably tends to be increased. As a result, the ratio of memory cost to the cost of the other hardware in the overall image processing apparatus is increased.
In order to reduce the required memory capacity, in general, the following techniques are employed: (1) a small-capacity memory is used in place of a bit map memory corresponding to the entire region of one printing sheet, and an image is printed in units of windows (bands), (2) a bit map is compressed by a compression technique, and (3) a bit map holds data at a low resolution, and data is enlarged while being smoothed in an output stage.
However, the prior art suffers from the following problems.
(1) When an image is printed in units of windows (bands), for example, when a complex figure such as graphic data is to be printed, a long period of time is required to develop the figure into windows, and window development cannot catch up with the output speed of a printing apparatus. As a result, a printing error may occur. PA1 (2) When a bit map is compressed, there is no satisfactory compression method for performing fixed-length reversible compression. As a variable-length compression method, entropy compression methods such as the MMR method and the like are known. However, since variable-length codes are used, data cannot be directly corrected. In addition, compression efficiency is decreased when a complex figure is to be compressed. PA1 (3) Image smoothing in an output stage inevitably causes deterioration of image quality as compared to a case wherein a high-resolution bit map is used, although this depends on the particular smoothing method.